It is common to provide on-chip transformers to provide transient voltage rejection. As the rate of change of the voltage (dV/dt) in the primary winding of the transformer increases the back electromagnetic field (EMF) increases. Thus, with increasing frequency, as is the case with voltage transients, the reactance increases and the windings act as low pass filters thereby rejecting the transient voltage. However, these on-chip transformers remain susceptible to external magnetic fields. As shown in FIG. 1, as the component of any external magnetic flux (H) 100 passes parallel to the coil axis it generates a current 102 in the coil 104. This effect is evident in both the primary and secondary windings of a transformer. One such prior art on-chip transformer is shown in FIG. 2, which shows a primary winding 200 formed by etching a metal layer to define a helical configuration. The transformer further includes a secondary winding 202, again defined by a helically etched metal layer. Typically these helical coils 200, 202 are formed by photolithographic techniques as known in the art. As will be appreciated from the discussion above, an external magnetic field 210 will generate current in both the secondary winding 202 and the primary 200. Thus, current will flow in the secondary winding 202 due not only to the current directly induced in the secondary winding by the magnetic field but also due to the magnetic coupling between the primary and secondary windings 200, 202, which causes the current induced in the primary winding 200 to be transferred to the secondary 202. It will therefore be appreciated that any external magnetic fields that pass through the transformer windings perpendicular to the windings (parallel to the axes of the windings, which in the transformer shown in FIG. 2 coincide with one another) will cause substantial interference.
In the field of electric guitars the vibration of the guitar strings is translated into sound by making use of pick-ups. These comprise coils formed around one or more permanent magnets. The permanent magnets define a magnetic field, which if altered, produces a current in the coils. By placing a pick-up underneath each of the guitar strings, the vibration of the string, which passes through the magnetic field of the magnet causes the reluctance to change, which produces a current in the coil that is then coupled though an amplifier to a speaker to produce sound. In U.S. Pat. Ser. No. 3,962,946 to Rickard, the use of two coils wound in opposite directions is described, with permanent magnet cores oriented in opposite directions (north-south in the one and south-north in the adjacent one). Thus the magnetic fields generated by the two adjacent magnets have flux lines flowing in opposite directions to produce currents flowing in opposite directions in the coils when the guitar string is plucked. However, since the coils themselves are oppositely wound and connected in series, the currents combine. On the other hand any external magnetic fields passing through the windings will cause currents generated in the two coils to cancel each other out to eliminate noise or hum due to interference from external magnetic fields such as the 60 Hz mains interference. The present invention seeks to reduce interference from external magnetic fields in an on-chip de-coupling transformer.